1. Field of the Invention:
This invention relates, in general, to electrical apparatus and, more specifically, to electrical apparatus for measuring electrical energy quantities occurring in an electrical power distribution system.
2. Description of the Prior Art:
In the field of electrical utility power and energy measurement, the amount of electrical power consumed by a load during the time T.sub.1 to T.sub.2 is determined by the following computation: ##EQU1## where e and i are the instantaneous magnitudes of the voltage and current, respectively, on the electrical power distribution system. The computation consists of time integrating the product of the instantaneous voltage and current.
Presently, the familiar electromechanical induction type watthour meter is used to measure the power consumed by an electrical load. In addition, watthour meters constructed with electronic and solid state devices have also been proposed. These devices employ various analog multiplying circuit techniques in order to compute the product of the voltage and current components of electrical energy. In one electronic power measuring circuit, shown in U.S. Pat. No. 3,764,908, which is assigned to the assignee of the present invention, the voltage and current signals are applied to a semiconductor device having logarithmetic multiplying capabilities which produces the product of the voltage and current signals. Similarly, U.S. Pat. No. 3,864,631, also assigned to the assignee of the present invention, utilizes a conventional analog multiplier circuit.
Another commonly used analog multiplier technique is known as time division multiplication. As shown in U.S. Pat. Nos. 4,056,775 and 3,794,917, a signal proportional to the voltage on the electrical distribution system is sampled to derive a pulse width modulated output signal having a pulse duration proportional to the amplitude of the voltage signal. This output signal controls a gating network which samples a current input signal at a rate proportional to the amplitude of the voltage signal. The resultant output is the product of the voltage and current and consists of a series of pulses having an amplitude proportional to the magnitude of the instantaneous current and a pulse width proportional to the magnitude of the voltage. After the product of the voltage and current signals is obtained, an output signal representative thereof is passed through an integrator to provide an resultant value proportional to the average power consumed by the load. The integrator output is applied to a voltage-to-frequency converter which provides a pulse train wherein each pulse is proportional to the total energy consumed. The pulse train is used to drive mechanical counters, magnetic recorders, or electrical counting circuits and thereby provides an indication of the total electrical power consumed by a load.
It has been found that the analog electronic multiplying techniques are sometimes difficult to apply in order to obtain the desired accuracies. Accurate, drift-free analog multipliers are often expensive and it is difficult to obtain square root computing circuits in the analog circuit field which are sometimes required for calculating electrical power quantities. Also, analog integration circuits required in the analog electronic power measuring apparatus produce undesired drift and variations over longtime intervals. In time division multiplication circuits, it is known that frequency dependent sampling occurs at the multiplier with the associated digital integration also having a similar dependency upon variations of the integrating capacitors.
It has also been proposed to utilize digital processing techniques to measure electrical energy parameters in an electrical distribution system. As shown in U.S. Pat. Nos. 3,569,785 and 3,984,737, the voltage and current signals are sampled at predetermined intervals and the resulting instantaneous values are converted to digital signals. The digital signals are processed by a digital computer which computes certain quantities required for relaying control functions. In U.S. Pat. Nos. 4,077,061 which is assigned to the assignee of the present invention, the voltage and current signals are sampled at random intervals and converted into digital form. The digital signals are then processed by a digital computer which computes the instantaneous values of electrical energy quantities and accumulates the instantaneous values of the computed quantity over a predetermined time interval to provide a measurement of the total watthours, Q.sup.2 hours and kilowatt demand by a particular load.
Although large size digital computers having sufficient speed and capacity to compute the necessary product of certain electrical quantities are available, their cost is prohibitive for use in electrical energy measuring apparatus, such as watthour meters. Smaller scale digital computers, while less costly, require many instructions and considerable time to perform the multiplication algorithm necessary to measure certain electrical energy quantities which, thereby, reduces the accuracy of the measuring apparatus below acceptable levels.
Thus, it is desirable to provide a meter apparatus for measuring certain electrical energy quantities occurring in an electrical distribution system which overcomes many of the shortcomings of electronic metering apparatus used in the prior art. It would also be desirable to provide an electrical energy measuring apparatus which retains the advantages of digital processing techniques and, at the same time, provides acceptable reliability and accuracy in measuring certain electrical energy parameters.